While Ext4 was originally merged in 2.6.19, it was marked as a development filesystem. It has been a long time coming but as planned, Ext4dev has been renamed to Ext4 in 2.6.28 to indicate its level of maturity and paving the way for production level deployments. Ext4 filesystem developer Ted Tso also endorsed Btrfs as a multi-vendor, next generation filesystem and along with the interest from Andrew Morton, Btrfs is planned to be merged before 2.6.29 is released. It will follow a similar development process to Ext4 and be initially marked as development only.

The on-disk format for ext3 is basically compatible with ext2. All it really does is adds a journal, and other features like extended attributes. An ext3 driver can mount an ext2 volume, and an ext2 driver can mount an ext3 volume.

I believe there are filesystem options that would render an ext3 volume unreadable by an ext2 driver, but I'm not 100% sure. If there are, they're obviously turned off by default.

The same is basically true of ext4, with one exception. Extents, which are enabled by default, change the on-disk format and render it incompatible with ext2 and ext3 drivers. If you turned those off, you should be able to read an ext4 volume with an ext2 driver. I'm not sure I'd want to risk writing to an ext4 filesystem with an ext2 driver though.